Recent scientific inquiry has focused upon an extracellular glycoprotein of Human Immunodeficiency Virus Type 1 (HIV-1), namely gp120. This glycoprotein is believed to play a vital role in attachment of HIV-1 to a host cell, by binding specifically to the host cell's cellular receptor CD4 and therefore important for the development of an AIDS vaccine. However, the gene encoding gp120 is highly variable among different HIV-1 strains, including sequential isolates from the same patient (1-4). The greatest variability has been demonstrated in several hypervariable regions which are flanked by more conserved regions (3, 4).
In response to the AIDS virus, the host immune system will produce antibodies targeted against various antigenic sites, or determinants, of gp120. Some of those antibodies will have a neutralizing effect and will inhibit HIV infectivity. It is believed that this neutralizing effect is due to the antibodies' ability to interfere with HIV's cellular attachment. It is also believed that this effect may explain in part, the rather long latency period between the initial seroconversion and the onset of clinical symptoms.
Much of our present knowledge about which domains of gp120 are immunogenic has come from studies of antibodies raised in laboratory animals. It was first observed that a variety of animals immunized with purified or recombinant gp120 or gp160 developed strain specific neutralizing antibodies (5-7). Subsequently, polyclonal animal antisera raised to recombinant or synthetic peptides representing the third hypervariable domain (V3) of gp120 (covering amino acid residues 307-330) were found to manifest similar strain specific neutralizing activity. This observation identified the V3 domain as a major target of neutralization, at least in laboratory animals (8,9). The V3 hypervariable region is flanked by conserved cysteine residues which may form a disulfide bond and define a "loop" region containing the largely conserved sequence Gly-Pro-Gly in its center. Synthetic loop region peptides have been found to elicit the production of antibodies that neutralize only virus obtained from isolates from which the synthetic peptide was derived. Hence, the V3 loop induces type-specific neutralizing antibodies; but these antibodies do not account for the broad virus-neutralizing activity detected in the sera of most infected persons.
Several type-specific neutralizing murine monoclonal antibodies have been produced that bind to epitopes within the V3 domain (10-12), allowing precise mapping of these epitopes. Murine monoclonal antibodies have also been generated to epitopes associated with the CD4 binding region near the COOH terminal of gp120 (14-15); but the extent that this region is also involved in neutralization has not yet been conclusively established. Although the peptide sequence in this region is relatively well conserved, Lasky et al (14) detected minor sequence differences in several virus strains. This finding has raised the possibility that some antigenic variation may also occur within the CD4 binding domain.
Our present knowledge about which epitopes of gp120 stimulate humoral responses in chronically infected humans is much more limited. In contrast to the type-specific neutralizing activity of animal antisera, sera of HIV infected patients generally contain more broadly reactive, group specific neutralizing-antibodies (15-17). Such broadly neutralizing antibodies may be directed against a conserved site on gp41, against antibodies specific for gag proteins (31-35), or possibly against conformational epitopes on gp120 (42). Nevertheless, several groups of investigators have also observed that some patient sera manifest distinct patterns of strain-restricted neutralizing activity when tested against a variety of strains (5,6,18,19). Moreover, patient antibodies that have undergone affinity purification to gp120 from one virus strain have been shown to possess type-specific neutralizing activity against that strain (6). These observations suggest that the multiplicity of antibodies commonly detected early in HIV-1 positive patient sera to variable epitopes may mask important later antibody responses to broad neutralization epitopes, which are believed to develop approximately six-twelve months after initial HIV seroconversion. Consequently, the need exists for the development of such agents with broad binding and neutralizing capabilities for the prevention, diagnosis and treatment of HIV and AIDS related infections. The present invention satisfies this need and provides related advantages as well.
The papers cited throughout this application are incorporated herein by reference.